The present invention relates to an arrangement in a silkscreen printing machine, and more particularly to an arrangement which indicates when the position of the material to be printed on the machine deviates from the intended position. The invention is intended for use with the type of silkscreen printing machine in which a printing drum is arranged to carry material which is to be printed. Cooperating with the drum is a stencil, which, during a printing operation, is pressed against the material by means of a so-called spueegee. As previously mentioned, the arrangement according to the invention is intended to indicate when the position of the aforesaid material during an actual printing operation deviates from a determined position. Thus, the arrangement shall be capable of indicating when the position assumed by the material in the gripping elements of the machine deviates from an exact predetermined position.
It has been found that particularly high requirements must be imparted to a printing drum used in a silkscreen printing machine of the above-mentioned type, where the printing drum is arranged for reciprocating or translatory movement. Such machines are also provided with a reciprocating movable frame in which the stencil is firmly secured, the movement of which frame is controlled by the movement of the drum. When the operating speed of such a silkscreen printing machine is increased, it is obvious that this increase in speed also produces an increase in kinetic energy. It has been found in practice that the increased kinetic energy imparted to the printing drum, the frame and other machine components, creates complicated problems. One such problem resides in the fact that the drum is unable to stop in exactly the same position each time to grip the material to be printed. This applies to a low kinetic energy level as well as to a high kinetic energy level. For example, at excessively high kinetic energy levels, the drum will over-rotate. One common answer to this problem is to lessen the weight of all of the machine elements. Although this solution might reduce the problem somewhat, it would not solve it completely.
The present invention contemplates a silkscreen printing machine having a reciprocating or translatory drum comprising means for indicating the position of the sheet material on the drum, thereby ensuring that the sheet is located in an exact predetermined position during a printing sequence.
The problem is not satisfactorily solved, however, merely by providing means in a silkscreen printing machine of the above type for indicating a deviated or displaced position of the sheet material which is held by the gripping elements. The indicating means must also be positioned in conjunction with gripping elements mounted on the drum, so as to permit the position of the sheet material in the gripping elements to be indicated. A fully satisfactory solution is achieved by having the leading edge of the sheet material engage a stop, so as to ensure that the leading edge assumes the exact position for engagement with the gripping elements. Furthermore, the stop should be designed to move away from the position in which it contacts the sheet material, immediately upon or after engagement of the gripping elements with the leading edge of the sheet material.
As previously mentioned, an increase in the speed of a silkscreen printing machine of the above type results in an increase in kinetic energy, which in turn causes the drum to stop not exactly in the same place each time to engage the sheet material to be printed. This holds true for low kinetic energy levels as well as for high kinetic energy levels, and at high speeds the drum will over-rotate. In order to position the leading edge of the material exactly in relation to the gripping elements, irrespective of the drum speed, the stop is mounted for movement to permit alignment of the leading edge with the position of the gripping elements.